This invention relates to a lamp output protective circuit for an electronic controller and, more particularly, to an overcurrent-protective circuit for a vehicle lamp output circuit.
In general, an electronic controller for automatic transmissions makes use of lamps to indicate a power mode, malfunctions, and the like. There are problems relating to such lamps as follows.
(1) In the case of a power mode display lamp, it is difficult for the driver to identify the power mode.
(2) In the case of a malfunction display lamp, the driver is liable to be unaware of a malfunction of the automatic transmission or the electronic controller.
A conventional technique in the field relating to these functions is known which will be described below.
FIG. 1 shows a conventional lamp output protective circuit for a lamp for the above kind of use.
This circuit is designed to constantly keep the thermal resistance of the filament of a lamp 1 large by allowing dark current to flow through a resistor 3, because in an initial stage of lighting of the lamp 1, the thermal resistance of the filament is small such that there is a possibility of destruction of a transistor 5. To protect the transistor 5 in the event of a short circuit of an output terminal 2, a resistor 4 is connected to the collector of the transistor 5.
A power supply of +5 V is connected to the base of the transistor 5 and to a signal line 7 of an electronic controller 8.
In this case, the resistor 3 has a resistance value of 560 .OMEGA., and a dark current of 17.6 mA flows, thereby maintaining the lamp with a large thermal resistance. The resistor 4 has a resistance value of 56 .OMEGA. whereby the maximum value of the collector current I.sub.C of the transistor 5 in the event of a short circuit between the output terminal 2 and a power supply of +12 V is limited to 214 mA.
In this conventional circuit, however, the effective resistance value of the resistor 4 is larger than a negligible value relative to the lamp resistance, and the illumination of the lamp is thereby reduced. There is also a problem of wasteful consumption of the battery energy due to the dark current flowing through the resistor 3. If the resistance of the lamp 1 during normal operation is 120 .OMEGA., the power consumption P.sub.L1 of the lamp is EQU P.sub.L .apprxeq.{120/(50.9+120).times.12}.sup.2 .times.1/120=0.592[W]
The combined resistance of the resistors 3 and 4 is EQU R=1/{1/56+1/560}=50.9[.OMEGA.]
and the power consumption of the lamp 1 is reduced to about 1/2 of the rating, resulting in a considerable reduction in the illumination.